fix: enable base URL and endpoint editing for Codex providers in edit mode

Fixed critical bug where CodexFormFields component was not rendered
in edit mode, preventing users from:
- Viewing and editing API base URL for custom/third-party Codex providers
- Accessing endpoint speed test modal in edit mode
- Updating API keys in edit mode

Changed line 477 from:
  {appType === "codex" && !isEditMode && (
To:
  {appType === "codex" && (

This aligns Codex behavior with Claude, where all form fields are
available in both create and edit modes for custom/third-party providers.

src/components/providers/forms/ProviderForm.tsx

@@ -474,7 +474,7 @@ export function ProviderForm({
         )}
 
         {/* Codex 专属字段 */}
-        {appType === "codex" && !isEditMode && (
+        {appType === "codex" && (
           <CodexFormFields
             codexApiKey={codexApiKey}
             onApiKeyChange={handleCodexApiKeyChange}